This invention relates to a zero-crossing type thyristor having a thyristor and a gate control element formed in the same semiconductor substrate, in a monolithic fashion, and more particularly to a thyristor having a gate control element of a thyristor structure.
A zero-crossing type thyristor is advantageously used, for the reason of reduced noise, as a power control switch provided at the final stage of a conventional temperature adjusting device, various industrial equipment, and the like. A zero-crossing thyristor can be triggered only when a voltage (anode-cathode voltage) applied between the main electrodes is within a specified narrow voltage range which includes a zero-crossing voltage. In most conventional zero-crossing thyristors, the gate control element is formed of a bipolar or MOS transistor structure such as is disclosed in Japanese Patent Disclosures No. 60-74678 and No. 60-149164.
The function of a prior art zero-crossing type thyristor will now be explained, with reference to FIGS. 8A and 8B.
FIG. 8A is a cross sectional view of the thyristor itself and FIG. 8B shows an equivalent circuit thereof. The thyristor has a MOSFET connected between the gate and cathode. Main thyristor 1 has a four layer structure which consists of P-type emitter layer 3, N-type base layer 4, P-type base layer 5, which is used as the gate, and N-type emitter layer 6. P-type layer 7, for extraction of a MOSFET driving voltage, is provided in main thyristor 1. Driving circuit 2 includes MOSFET 9 and zener diode 10 for protection of the gate oxide film formed in P-well region 8. A, K, and G respectively represent the anode, cathode, and gate electrodes of the composite thyristor.
There will now be described the operation of the thyristor wherein a forward bias voltage Vak (&gt;0) is applied between electrodes A and K. P-type layer 7 is connected to the anode and cathode electrodes via respective electrostatic capacitors. However, since the capacitor of the former is larger than that of the latter, the potential of P-type layer 7 is set substantially equal to that of the anode electrode. In this case, when a depletion layer created between P-well region 8 and N-type base layer 4 has reached N-type base layer 7 with the increase in Vak, the value of Vak is saturated.
The gate threshold voltage of MOSFET 9 is set at, for example, 5 V. When Vak is higher than 5 V, MOSFET 9 is turned on so as to short-circuit a path between electrodes G and K of the thyristor. This causes an externally supplied trigger signal to be bypassed via MOSFET 9, preventing the thyristor from being triggered. With Vak is in the range of 0 to 5 V, MOSFET 9 is turned off so as to interrupt the short circuit path between G and K electrodes. Thus, the thyristor can be triggered when Vak is at a voltage near 5 V or within a specified narrow voltage range. When an A.C. power source voltage of commercial frequency is supplied intermittently, those prior art thyristors other than the zero-crossing type often tend to be triggered in a high A.C. voltage phase. If at this time, the load of the thyristor is reactive, the excessive voltage will give rise to a rush current or else noise, which causes LSI circuits, IC logic circuits and the like to malfunction, and causes electromagnetic disturbance in electronic equipment such as radios and televisions. Zero-crossing type thyristors were developed for the purpose of eliminating this problem, and, as such, are increasingly in demand, their field of application rapidly widening.